Tablets in which graphic and character recognition and command execution can be carried out by instructions that are given with a pen, finger or other means have been known in the art as one type of input devices that carry out predetermined input operations to computers, word processors, and other apparatuses. Moreover, Japanese Laid-Open Patent Publication No. 53726/1993 (Tokukaihei 5-53726) and Japanese Laid-Open Patent Publication No. 314166/1994 (Tokukaihei 6-314166) disclose display-integrated type tablet devices which achieve inexpensive constructions by commonly using electrode constructions and driving circuits in liquid crystal panels and other components, which are regarded as further advanced tablets.
It is required for such coordinate input devices to carry out coordinate detection with high precision at high speeds. In recent years, pixels in the display device become smaller and smaller, and such fine display has further increased the demands for high precision of the coordinate detection.
However, high speed and high precision of the coordinate input device form two effects that are inconsistent with each other; therefore, the problem with the prior art coordinate input device is that the coordinate detecting precision can not be improved with high-speed coordinate detection maintained as it is. In other words, high-speed coordinate detection needs to widen the frequency band width, resulting in degradation in the precision.
Among these coordinate input devices, especially in the display-integrated type devices, the coordinate detecting precision further deteriorate for the following reasons: For example, the arrangement of the above-mentioned Japanese Laid-Open Patent Publication No. 53726/1993 (Tokukaihei 5-53726) is designed to provide the display and coordinate detection functions by carrying out the image display and coordinate detection in a time-sharing manner. In this case, the coordinate detection period is set as short as possible in order for the coordinate detection operation not to give adverse effects on the display. Further, the applied voltage used for detection in not increased so much since it is limited by the resistance to voltage of the display driver. Moreover, in the case of duty-type liquid crystal, since this system utilizes the electrostatic coupling that is exerted between the tip electrode of an electronic pen and the transparent electrode group on one side through the gap of the electrode group on the other side, only a minute detection voltage is obtained. Consequently, the above-mentioned display-integrated type tablet devices only provide a coordinate detection signal with an inferior S/N ratio.
Moreover, the above-mentioned Japanese Laid-Open Patent Publication No. 314166/1994 (Tokukaihei 6-314166) shows an arrangement wherein a tablet function is added to a TFT (Thin Film Transistor) liquid crystal panel using the addressing system. In order to improve display quality, a present trend in TFT liquid crystal panels is to improve the aperture ratio by narrowing the widths of gate electrodes and source electrodes so as to widen the areas of pixel electrodes. In this case also, since the coordinate detection signal is detected in proportion to the coupling capacitance between the tip electrode of an electronic pen and the gate electrodes or the source electrodes, the coupling capacitance decreases with the improvement of the aperture ratio, thereby resulting in degradation in the S/N ratio of the detection signal. Here, in the TFT liquid crystal panels also, it is needless to say that the coordinate detection period should be maintained as short as possible so as not to give adverse effects on the display.
Furthermore, in the prior art coordinate input devices, it is essential to provide a connecting line for the electronic pen so as to supply power to the electronic pen and to output the coordinate detection signal. Here, this connecting line is a disturbing object for the user to operate the electronic pen, causing degradation in the operability.
Additionally, another tablet using the pressure-sensitive system, which has two superimposed sheets of transparent electrodes with an insulating spacer inserted therebetween and which gives specified coordinates by using a current that flows between the detection electrodes when pressed by a pen point or a finger, has been proposed. However, the problems with this system are that merely an inferior visibility is obtained due to the transmittance and reflection of the transparent electrodes and that a finger and a pen cannot be used at the same time.
Further, U.S. Pat. No. 4,639,720 has proposed an arrangement wherein a pen-input function is achieved by intersecting transparent electrodes in liquid crystal so as to measure the electrostatic coupling capacitance between them, a pen point or a finger. However, this arrangement has a deficiency in that a complicated structure and driving method are required since the liquid crystal electrodes have to be designed to intersect each other on the same plane.